Three far-right activists in France have been charged with aggravated violence after being exposed in an undercover investigation by Al Jazeera.

The three men, who were arrested and charged last week in the northern city of Lille, are accused of involvement in an assault filmed by an undercover reporter for Al Jazeera's Investigative Unit.

A fourth activist has been released without charge, according to the Lille prosecutor's office.

The attack was broadcast in a two-part documentary, Generation Hate, released last December.

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The prosecutor's office did not reveal the names of the three men who were charged.

A local TV station, France 3, however, named one of them as Remi Falize, a leading figure at the Flanders branch of the far-right group Generation Identity.

The branch headquarters is at the Citadelle bar in Lille city centre.

The documentary shows Falize, wearing gloves reinforced with plastic, striking a young woman four times on the head after she is heard using Arabic slang.

Following the assault, Falize was filmed boasting about the attack and suggesting a racist motive. Girl or not, they're just Arabs, he says.

The three men are due to appear in court on May 10.

They have also been banned from going to Rue Masséna - where the alleged assault took place - from meeting the victim, and from meeting the other two men who have also been charged.

Meanwhile, Aurelien Verhassel, former leader of Generation Identity in Lille and president of the Citadelle organisation, was questioned by police for four hours last week.

La Voix du Nord newspaper reported that Verhassel was questioned about footage in the documentary in which he is seen brandishing a flash-ball - a riot-control gun used by police - at the Citadelle. Verhassel was released without charge.

The two documentaries, released on December 9 and 16, exposed violence and racism at the heart of Generation Identity.

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The investigation, which included a six-month undercover operation, also revealed evidence of close links between Identitarian activists and key figures in Marine Le Pen's National Front party, which has since changed its name to the National Rally.

In the documentary, two members of the European Parliament, Christelle Lechevalier and Sylvie Goddyn, are seen visiting the Citadelle and expressing support for Generation Identity.

Martine Aubry, the Socialist Party mayor of Lille, expressed shock at the Al Jazeera investigation's findings and called for the Citadelle to be shut down.

Generation Identity's national spokesperson publicly distanced the organisation from Verhassel, claiming that he was no longer a member of their movement. Verhassel, however, insists he is still a member.

In a recent press conference, Verhassel claimed that Al Jazeera's documentary was based on "transient visitors" to the Citadelle who were not linked to the group's "activist base".

In response, Al Jazeera's Investigative Unit released new evidence, including unreleased footage and internal group discussion messages, which suggested that Remi Falize and Charles Tessier were key activists at the Citadelle.